3GPP LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme. In radio communication systems to which 3GPP LTE is applied, base stations transmit synchronization signals (i.e., Synchronization Channel: SCH) and broadcast signals (i.e., Broadcast Channel: BCH) using predetermined communication resources. Meanwhile, each terminal finds an SCH first and thereby ensures synchronization with a base station. Subsequently, the terminal reads BCH information to acquire base station-specific parameters (see, Non-Patent Literatures (hereinafter, abbreviated as NPL) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameters, each terminal sends a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via Physical Downlink Control CHannel (PDCCH) as appropriate to the terminal with which a communication link has been established.
The terminal performs “blind-determination” on each of a plurality of pieces of control information included in the received PDCCH signals (i.e., Downlink (DL) Assignment Control Information: also referred to as Downlink Control Information (DCI)). To put it more specifically, each piece of the control information includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part by the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the received piece of control information with its own terminal ID, the terminal cannot determine whether or not the piece of control information is intended for the terminal. In this blind-determination, if the result of demasking the CRC part indicates that the CRC operation is OK, the piece of control information is determined as being intended for the terminal.
Moreover, in 3GPP LTE, Automatic Repeat Request (ARQ) is applied to downlink data to terminals from a base station. To put it more specifically, each terminal feeds back response signals indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as response signals. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to feed back the response signals (i.e., ACK/NACK signals (hereinafter, may be referred to as “A/N,” simply)).
The control information to be transmitted from a base station herein includes resource assignment information including information on resources assigned to the terminal by the base station. As described above, PDCCH is used to transmit this control information. The PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH consists of one or more Control Channel Elements (CCE). To put it more specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH consists of a plurality of CCEs (2, 4 or 8), a plurality of contiguous CCEs starting from a CCE having an even index are assigned to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource assignment target terminal in accordance with the number of CCEs required for notifying the control information to the resource assignment target terminal. The base station maps the control information to physical resources corresponding to the CCEs of the L1/L2 CCH and transmits the mapped control information.
In addition, CCEs are associated with component resources of PUCCH (hereinafter, may be referred to as “PUCCH resource”) in a one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the component resources of PUCCH that correspond to the CCEs forming the L1/L2 CCH and transmits response signals to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the response signals to the base station using a PUCCH component resource corresponding to a CCE having a smallest index among the plurality of PUCCH component resources respectively corresponding to the plurality of CCEs (i.e., PUCCH component resource associated with a CCE having an even numbered CCE index). In this manner, the downlink communication resources are efficiently used.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristic of zero autocorrelation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in a PUCCH. In FIG. 1, (W0, W1, W2, W3) represent a length-4 Walsh sequence and (F0, F1, F2) represent a length-3 DFT sequence. As illustrated in FIG. 1, ACK or NACK response signals are primary-spread over frequency components corresponding to 1 SC-FDMA symbol by a ZAC sequence (length-12) in frequency-domain. To put it more specifically, the length-12 ZAC sequence is multiplied by a response signal component represented by a complex number. Subsequently, the ZAC sequence serving as the response signals and reference signals after the primary-spread is secondary-spread in association with each of a Walsh sequence (length-4: W0-W3 (may be referred to as Walsh Code Sequence)) and a DFT sequence (length-3: F0-F2). To put it more specifically, each component of the signals of length-12 (i.e., response signals after primary-spread or ZAC sequence serving as reference signals (i.e., Reference Signal Sequence) is multiplied by each component of an orthogonal code sequence (i.e., orthogonal sequence: Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into signals of length-12 in the time-domain by inverse fast Fourier transform (IFFT). A CP is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
The response signals from different terminals are spread using ZAC sequences each corresponding to a different cyclic shift value (i.e., index) or orthogonal code sequences each corresponding to a different sequence number (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, an orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed plurality of response signals using the related art despreading and correlation processing (see, NPL 4).
However, it is not necessarily true that each terminal succeeds in receiving downlink assignment control signals because the terminal performs blind-determination in each subframe to find downlink assignment control signals intended for the terminal. When the terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal would not even know whether or not there is downlink data intended for the terminal on the downlink component carrier. Accordingly, when a terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal generates no response signals for the downlink data on the downlink component carrier. This error case is defined as discontinuous transmission of ACK/NACK signals (DTX of response signals) in the sense that the terminal transmits no response signals.
In 3GPP LTE systems (may be referred to as “LTE system,” hereinafter), base stations assign resources to uplink data and downlink data, independently. For this reason, in the 3GPP LTE system, terminals (i.e., terminals compliant with LTE system (hereinafter, referred to as “LTE terminal”)) encounter a situation where the terminals need to transmit uplink data and response signals for downlink data simultaneously in the uplink. In this situation, the response signals and uplink data from the terminals are transmitted using time-division multiplexing (TDM). As described above, the single carrier properties of transmission waveforms of the terminals are maintained by the simultaneous transmission of response signals and uplink data using TDM.
In addition, as illustrated in FIG. 2, the response signals (i.e., “A/N”) transmitted from each terminal partially occupy the resources assigned to uplink data (i.e., Physical Uplink Shared CHannel (PUSCH) resources) (i.e., response signals occupy some SC-FDMA symbols adjacent to SC-FDMA symbols to which reference signals (RS) are mapped) and are thereby transmitted to a base station in time-division multiplexing (TDM). In FIG. 2, however, “subcarriers” in the vertical axis of the drawing are also termed as “virtual subcarriers” or “time contiguous signals,” and “time contiguous signals” that are collectively inputted to a discrete Fourier transform (DFT) circuit in a SC-FDMA transmitter are represented as “subcarriers” for convenience. To put it more specifically, optional data of the uplink data is punctured due to the response signals in the PUSCH resources. Accordingly, the quality of uplink data (e.g., coding gain) is significantly reduced due to the punctured bits of the coded uplink data. For this reason, base stations instruct the terminals to use a very low coding rate and/or to use very large transmission power so as to compensate for the reduced quality of the uplink data due to the puncturing.
Meanwhile, the standardization of 3GPP LTE-Advanced for realizing faster communications than 3GPP LTE has started. 3GPP LTE-Advanced systems (may be referred to as “LTE-A system,” hereinafter) follow 3GPP LTE systems (may be referred to as “LTE system,” hereinafter). 3GPP LTE-Advanced is expected to introduce base stations and terminals capable of communicating with each other using a wideband frequency of 40 MHz or greater to realize a downlink transmission rate up to 1 Gbps or above.
In the LTE-A system, in order to simultaneously achieve backward compatibility with the LTE system and ultra-high-speed communications several times faster than transmission rates in the LTE system, the LTE-A system band is divided into “component carriers” of 20 MHz or below, which is the bandwidth supported by the LTE system. In other words, the “component carrier” is defined herein as a band having a maximum width of 20 MHz and as the basic unit of communication band. Moreover, “component carrier” in downlink (hereinafter, referred to as “downlink component carrier”) is defined as a band obtained by dividing a band according to downlink frequency bandwidth information in a BCH broadcasted from a base station or as a band defined by a distribution width when a downlink control channel (PDCCH) is distributed in the frequency domain. In addition, “component carrier” in uplink (hereinafter, referred to as “uplink component carrier”) may be defined as a band obtained by dividing a band according to uplink frequency band information in a BCH broadcasted from a base station or as the basic unit of a communication band of 20 MHz or below including a Physical Uplink Shared CHannel (PUSCH) in the vicinity of the center of the bandwidth and PUCCHs for LTE on both ends of the band. In addition, the term “component carrier” may be also referred to as “cell” in English in 3GPP LTE-Advanced.
The LTE-A system supports communications using a band obtained by aggregating several component carriers, so called “carrier aggregation.” In general, throughput requirements for uplink are different from throughput requirements for downlink. For this reason, so called “asymmetric carrier aggregation” has been also discussed in the LTE-A system. In asymmetric carrier aggregation, the number of component carriers configured for any terminal compliant with the LTE-A system (hereinafter, referred to as “LTE-A terminal”) differs between uplink and downlink. In addition, the LTE-A system supports a configuration in which the numbers of component carriers are asymmetric between uplink and downlink, and the component carriers have different frequency bandwidths.
FIG. 3 is a diagram provided for describing asymmetric carrier aggregation and a control sequence applied to individual terminals. FIG. 3 illustrates a case where the bandwidths and numbers of component carriers are symmetric between the uplink and downlink of base stations.
In FIG. 3, a configuration in which carrier aggregation is performed using two downlink component carriers and one uplink component carrier on the left is set for terminal 1, while a configuration in which the two downlink component carriers identical with those used by terminal 1 are used but uplink component carrier on the right is used for uplink communications is set for terminal 2.
Referring to terminal 1, an LTE-A base station and an LTE-A terminal included in the LTE-A system transmit and receive signals to and from each other in accordance with the sequence diagram illustrated in FIG. 3A. As illustrated in FIG. 3A, (1) terminal 1 is synchronized with the downlink component carrier on the left when starting communications with the base station and reads information on the uplink component carrier paired with the downlink component carrier on the left from a broadcast signal called system information block type 2 (SIB2). (2) Using this uplink component carrier, terminal 1 starts communications with the base station by transmitting, for example, a connection request to the base station. (3) Upon determining that a plurality of downlink component carriers need to be assigned to the terminal, the base station instructs the terminal to add a downlink component carrier. However, in this case, the number of uplink component carriers is not increased, and terminal 1, which is an individual terminal, starts asymmetric carrier aggregation.
In addition, in the LTE-A system to which carrier aggregation is applied, a terminal may receive a plurality of pieces of downlink data on a plurality of downlink component carriers at a time. In LTE-A, studies have been carried out on channel selection (also referred to as “multiplexing”), bundling and a discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) format as a method of transmitting a plurality of response signals for the plurality of pieces of downlink data. In channel selection, not only symbol points used for response signals, but also the resources to which the response signals are mapped are varied in accordance with the pattern of results of the error detection on the plurality of pieces of downlink data. Compared with channel selection, in bundling, ACK or NACK signals generated according to the results of error detection on the plurality of pieces of downlink data are bundled (i.e., bundled by calculating a logical AND of the results of error detection on the plurality of pieces of downlink data, provided that ACK=1 and NACK=0), and response signals are transmitted using one predetermine resource. In transmission using the DFT-S-OFDM format, a terminal jointly encodes (i.e., joint coding) the response signals for the plurality of pieces of downlink data and transmits the coded data using the format (see, NPL 5).
To put it more specifically, channel selection is a technique that varies not only the phase points (i.e., constellation points) of the response signals but also the resources used for transmission of the response signals on the basis of whether the response signals for the plurality of pieces of downlink data received on the plurality of downlink component carriers are ACK or NACK as illustrated in FIG. 4. Meanwhile, bundling is a technique that bundles ACK/NACK signals for the plurality of pieces of downlink data into a single set of signals and thereby transmits the bundled signals using one predetermined resource (see, NPLs 6 and 7).
A description will be herein provided regarding ARQ control using channel selection and bundling in a case where asymmetric carrier aggregation is applied to terminals, with reference to FIG. 4.
For example, as illustrated in FIG. 4, when a component carrier group consisting of downlink component carriers 1 and 2 and uplink component carrier 1 (may be referred to as “component carrier set” in English) is configured for terminal 1, downlink resource assignment information is first transmitted from a base station to terminal 1 on each of the PDCCHs of respective downlink component carriers 1 and 2, and downlink data is then transmitted using the resource corresponding to the downlink resource assignment information.
In channel selection, when a terminal succeeds in receiving the downlink data on component carrier 1 but fails to receive the downlink data on component carrier 2 (i.e., the response signals of component carrier 1 are ACK and the response signals of component carrier 2 are NACK), the response signals are mapped to a PUCCH resource in PUCCH region 1 and a first phase point (e.g., the phase point (1,0) and/or the like) is used as the phase point of the response signals. In addition, when a terminal succeeds in receiving the downlink data on component carrier 1 and succeeds in receiving the downlink data on component carrier 2, the response signals are mapped to a PUCCH resource in PUCCH region 2 and the first phase point is used. To put it more specifically, when there are two downlink component carriers, the results of error detection are represented in four patterns, and the four patterns can be represented by the combinations of two resources and two types of phase points.
In bundling, when succeeding in receiving both of the two pieces of downlink data (CRC=OK), terminal 1 calculates a logical AND of ACK (=1) for downlink component carrier 1 and ACK (=1) for downlink component carrier 2 and transmits the result of calculation, which is “1” (i.e., ACK), to the base station as bundled ACK/NACK signals. Meanwhile, when succeeding in receiving the downlink data on downlink component carrier 1 but failing to receive the downlink data on downlink component carrier 2, terminal 1 calculates a logical AND of ACK (=1) for downlink component carrier 1 and NACK (=0) for downlink component carrier 2 and transmits the result of calculation, which is “0” (i.e., NACK), to the base station as bundled ACK/NACK signals. Likewise, when failing to receive both of the pieces of downlink data, terminal 1 calculates a logical AND of NACK (=0) and NACK (=0) and feeds back “0” (i.e., NACK) to the base station as bundled ACK/NACK signals.
As described above, in bundling, only when succeeding in receiving all of the plurality of pieces of downlink data transmitted to the terminal, the terminal transmits only one ACK to the base station as bundled ACK/NACK signals, and when failing to receive even one piece of downlink data, the terminal transmits only one NACK to the base station as bundled ACK/NACK signals. In this manner, the overhead of the uplink control channels can be reduced. It should be noted that, each terminal transmits bundled ACK/NACK signals using a PUCCH resource having the lowest frequency or identification number (i.e., index), for example, among the PUCCH resources corresponding to the plurality of CCEs that have been occupied by the received plurality of downlink control signals.
Next, a description will be provided regarding a method of transmitting bundled ACK/NACK signals using the DFT-S-OFDM format with reference to FIG. 5. The coded data obtained by jointly encoding (i.e., joint coding) the response signals for the plurality of pieces of downlink data transmitted using the DFT-S-OFDM format includes the results of error detection for the respective downlink component carriers as individual pieces of data. The coded data that is obtained by jointly encoding (i.e., joint coding) the response signals for the plurality of pieces of downlink data and that includes the results of error detection for the respective downlink component carriers is hereinafter referred to as “bundled ACK/NACK signals” or “bundled response signals.”
As the reference signals used for demodulating the bundled ACK/NACK signals, a “length-12 ZAC sequence (i.e., base sequence)” similar to the reference signals in LTE is used. To put it more specifically, a length-12 ZAC sequence is placed on the second and sixth SC-FDMA symbols and secondary-spread in association with a Walsh sequence (length-2: W′0, W′1). In addition, the spread signals are transformed into time-domain signals by IFFT. The processing described above is equivalent to the processing in which the signals obtained by transforming the ZAC sequence into the time-domain signals using IFFT processing is spread using a length-2 Walsh sequence.
As in the case of reference signals for ACK/NACK in LTE, the reference signals from different terminals are spread using sequences each corresponding to a different cyclic shift value (i.e., cyclic shift index) or a different Walsh sequence. Thus, base stations can demultiplex the plurality of code-multiplexed reference signals using the related art despreading and correlation processing.
In the DFT-S-OFDM format illustrated in FIG. 5, a “length-12 ZAC sequence” is used as the reference signals as described above. In this case, the signals consisting of 12 symbols are subjected to DFT processing and then primary-spread in 1 SC-FDMA symbol as bundled ACK/NACK signals. As described above, the response signals of one symbol obtained by BPSK modulation are primary-spread in 1 SC-FDMA symbol using a ZAC sequence (of length-12) in the frequency-domain in the LTE system. In contrast to the LTE system, when a “length-12 ZAC sequence” is used as the reference signals for reporting bundled ACK/NACK signals using DFT-S-OFDMA in the LTE-A system to which carrier aggregation is applied, the bundled ACK/NACK signals consisting of 12 symbols are subjected to DFT processing and primary-spread in 1 SC-FDMA symbol. It should be noted that, the bundled ACK/NACK signals consisting of 12 symbols include the results of error detection for the respective component carriers as individual pieces of data as described above.
Subsequently, the bundled ACK/NACK signals that have undergone DFT processing are placed on the first, third, fourth, fifth and seventh SC-FDMA symbols and spread in association with a DFT sequence (of length-5: F′0, F′1, F′2, F′3, F′4). Moreover, the spread signals are transformed into the time-domain signals by IFFT processing. The processing described above is equivalent to the processing in which the signals obtained by transformation into the time-domain signals using IFFT processing are multiplied by each component element of the length-5 DFT sequence.
The bundled ACK/NACK signals from different terminals are code-multiplexed herein by spreading the bundled ACK/NACK signals using different DFT sequences. To put it more specifically, since the bundled ACK/NACK signals are spread using the length-5 DFT sequence, bundled ACK/NACK signals from as many as five terminals can be code-multiplexed.
In addition, a cyclic prefix (CP) is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
Hereinafter, the resources adopting the DFT-S-OFDM format structure and used for transmitting the bundled ACK/NACK signals are referred to as “bundled ACK/NACK resource.” As illustrated in FIG. 5, when downlink data is transmitted using the DFT-S-OFDM format, the bundled ACK/NACK signals are placed on the data parts where downlink data is placed (i.e., first, third, fourth, fifth and seventh SC-FDMA symbols in the example in FIG. 5). In addition, the reference signals for demodulating the bundled ACK/NACK signals are time-multiplexed with the bundled ACK/NACK signals.
Moreover, the introduction of radio communication relay apparatuses (hereinafter, referred to as “relay station” or “RN: relay node”) is set forth for the purpose of achieving an increase in the coverage in LTE-A (see, FIG. 6). Along with the introduction of relay stations, the standardization of downlink control channels from base stations to relay stations (hereinafter, referred to as “R-PDCCH”) is in progress (e.g., see, NPLs 8, 9, 10 and 11). Currently, the following matters on R-PDCCH are discussed. FIG. 7 illustrates R-PDCCH regions.
(1) The mapping start position of an R-PDCCH in the time-domain direction is fixed to the fourth OFDM symbol from the top OFDM symbol in a single subframe. This position is fixed independently of the proportion of symbols occupied by a PDCCH in the time-domain direction.
(2) As a method of mapping an R-PDCCH in the frequency-domain direction, two assignment methods (i.e., distributed and localized methods) are supported.
(3) As the reference signals for demodulation, common reference signals (CRS) and demodulation reference signals (DM-RS) are supported. Base stations notify relay stations of which reference signals are used.